Ion-exchange membranes have numerous applications. In particular, proton-exchange membranes (PEMs) are used in fuel cells, and such applications have caused great interest in proton-conducting polymers.
Ideally, a PEM has high proton conductivity, low electron conductivity, dimensional stability (low swelling in water), oxidative, reductive and hydrolytic stability, and low fuel cross-over rates (e.g. low methanol cross-over for direct methanol fuel cells, or low hydrogen or nitrogen cross-over for H2/O2 fuel cells). To date, reported membranes that conduct protons at moderate temperature also possess high methanol and hydrogen permeability and suffer from poor dimensional stability.
The great tendency of a PEM to swell in hot water has led to development of cross-linking methods with the aim of improving mechanical stability. Buchi et al. (J. Electrochem. Soc., 142, 3044 (1995)) reported materials synthesized using a sulfonated cross-linked polyolefin-polystyrene copolymer. The polymer is cross-linked during polymerization by the addition of divinyl benzene in the presence of radical initiator.
International Patent Application WO 97/019480 to Yen et al. discloses that sulfonated polymers will form direct bonds therefore cross-linking the polymer chains. However, this reaction decreases the ion exchange capacity and therefore the proton conductivity.
U.S. Pat. No. 6,090,895 to Mao et al. discloses partially cross-linked polymers through activation of the sulfonic acids groups via conversion to the sulfonic acid chloride/bromide with subsequent reaction with aromatic/aliphatic diamines. This sulfonamide cross-linking is not sufficiently stable to hydrolysis for fuel cell application and use of the sulfonic acid groups decreases the ion exchange capacity and therefore the proton conductivity.
International Patent Application WO 00/072395 to Pintauro et al. discloses the cross-linking of sulfonated polyphosphazene in the presence of ultraviolet light and benzophenone. U.S. Patent Publication No. 2004/0116546 to Kosek et al. discloses imbibing a perfluorocarbon membrane with styrene monomer and vinyl benzene to yield a cross-linked polymer within the perfluorocarbon membrane.
However, the disclosed polymers fail to provide desired properties, particularly in relation to fuel cell applications. Reported membranes possess high methanol and hydrogen permeability, and suffer poor dimensional stability.